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Abstract

The ability to control chromatic dispersion is paramount in applications where the optical pulsewidth is critical, such as chirped pulse amplification and fiber optic communications. Typically, devices used to generate large amounts (>100 ps/nm) of chromatic dispersion are based on diffraction gratings, chirped fiber Bragg gratings, or dispersion compensating fiber. Unfortunately, these dispersive elements suffer from one or more of the following restrictions: (i) limited operational bandwidth, (ii) limited total dispersion, (iii) low peak power handling, or (iv) large spatial footprint. Here, we introduce a new type of tunable dispersive device, which overcomes these limitations by leveraging the large modal dispersion of a multimode waveguide in combination with the angular dispersion of diffraction gratings to create chromatic dispersion. We characterize the device’s dispersion, and demonstrate its ability to stretch a sub-picosecond optical pulse to nearly 2 nanoseconds in 20 meters of multimode optical fiber. Using this device, we also demonstrate single-shot, time-wavelength atomic absorption spectroscopy at a repetition rate of 90.8 MHz.

Figures (5)

Schematic diagram of the chromo-modal dispersion (CMD) device. Light is incident upon two parallel plane gratings, which disperse and collimate the optical spectrum. Angular dispersion is then applied to the spectrum using a lens. The input facet of a multimode fiber is placed at the lens focus such that the various spectral components are coupled into different fiber modes. The figure inset illustrates how different spectral components are coupled into and propagate in the multimode fiber. The dashed line represents the optic axis of the fiber. Although the configuration shown provides anomalous dispersion, the CMD can be tuned throughout both the anomalous and normal dispersive regimes by adjusting the alignment of the lens and fiber relative to the spatially dispersed spectrum.

(a) Relative delay and (b) dispersion parameter for three different alignments of the CMD. Translating the lens and fiber facet in the horizontal direction relative to the incident spatially dispersed spectrum shifts the CMD’s zero dispersion wavelength.

Mode profiles at the output of the CMD fiber for alignment position #1, measured 5 mm from the output fiber facet. (a) Mode profiles recorded using various excitation wavelengths. (b) Mode profile using 745-nm excitation after propagating through 20 m of 200-μm core diameter fiber. (c) Mode profile using 745-nm excitation after propagating through 35 m of the same fiber. Blurring in the output mode profile indicates the coupling length has been reached. The scale bar in all cases is 1 mm.

Time-domain profile and spectrum of an 80-fs optical pulse after propagation through the CMD. (a) Spectrum of the input optical pulse and the CMD dispersion parameter. (b) Measured time-domain output (red) and calculated time-domain profile for the measured input spectrum and dispersion curve.

Single-shot atomic absorption spectroscopy of rubidium vapor. (a) Optical spectrum after passing though the rubidium cell and the CMD, as measured by a conventional spectrometer. The arrow represents the position of the Doppler-broadened D2-absorption line. (b) Sequence of four consecutive single-shot time-domain traces captured by a photodiode and oscilloscope at a repetition rate of 90.8 MHz. High-frequency amplitude noise in the time-domain signals is a result of ringing in the measurement, and is not fundamental to the time-wavelength spectroscopy technique. The horizontal time axis can be converted to wavelength with knowledge of the CMD dispersion curve. Peaks “1” and “2” are denoted in both (a) and (b) to illustrate correspondence in the time-wavelength mapping.